Patients with traumatic peripheral nerve or spinal cord injuries (250,000 Americans alone) require extensive medical treatment. How to coax injured axons to re-grow to their appropriate synaptic targets remains an important unsolved problem, whether applied to damaged neurons or neural stem cells. Despite their regenerative potential, injuries to peripheral nerves frequently lack a favorable functional outcome and remain an important clinical problem. Immediately after nerve injury, an active process of axonal degeneration, termed Wallerian degeneration, ensues. The goal of this proposal is to understand the underlying mechanisms controlling Wallerian degeneration, axon regeneration, and the re-establishment of functional connections following nerve injury. The zebrafish peripheral nerve is an ideal model to study the genetic, cellular and molecular mechanisms underlying degeneration and regeneration in vivo. Zebrafish motor nerves are easily accessible, and precise axonal lesions can be reproducibly induced by laser and the ensuing process of axon degeneration can be imaged in live intact animals over several days. Zebrafish axons, like those of vertebrate mammals, undergo Wallerian degeneration upon injury. Wallerian degeneration also occurs in degenerative pathologies of the peripheral and central nervous systems, such as Amyotrophic Lateral Sclerosis and Spinal Muscular Atrophy. The importance of axon degeneration is highlighted by how its speed and efficiency determine the success of regeneration in the Peripheral Nervous system, whereas its slowness and inefficacy prevent regeneration in the Central Nervous system. Despite the critical role of Wallerian degeneration in laying the foundation for regeneration, the cellular and molecular basis of these processes in vivo is not well understood, due in part to the intrinsic difficulties of live cell imaging in mammalian model systems. In this proposal I will first investigate intracellular changes within the axon during Wallerian degeneration to determine whether changes in mitochondria or calcium contribute to or cause the sudden fragmentation that is characteristic of Wallerian degeneration. I will also investigate when axonal microtubules degenerate, a critical commitment step in axonal degeneration. Second, I will determine how local cell types such as Schwann cells, macrophages, and perineural ensheathing glia contribute to the process of axon degeneration by lesioning axons in the absence of each of these cell types using various mutant animals. Lastly, I will screen a library of known bioactive compounds to identify molecular mechanisms that can enhance degeneration and regeneration in vivo.